A multi-layer laminated film consists of alternating low-refractive index layers and high-refractive index layers and can be used as an optical interference film which selectively reflects or transmits light having a specific wavelength by structural optical interference between these layers. When this multi-layer laminated film is changed in layer thickness gradually or assembled with a film having a different reflection peak, it can achieve a reflectance as high as that of a film made of a metal and can be used as a metal glossy film or reflection mirror. Further, it is known that when this multi-layer laminated film is stretched in only one direction, it can be used as a polarization reflection film which reflects only a specific polarization component and also that when this stretched film is used in a liquid crystal display, it can be used as a brightness improving film for liquid crystal displays.
In general, a phenomenon such as “increased reflection” that light having a specific wavelength is reflected according to a difference in refractive index between two layers, the thickness of each layer and the number of layers is seen in a multi-layer optical film consisting of layers having a thickness of 0.05 to 0.5 μm and different refractive indices. The reflection wavelength is generally represented by the following equation (1).λ=2x((n1)×(d1)+(n2)×(d2))  (1)wherein λ is a reflection wavelength (nm), n1 and n2 are therefractive indices of the respective layers, and d1 and d2 are thicknesses of the respective layers (nm).
As shown in JP-A 04-268505, when a resin having a positive stress optical coefficient is used in one layer, anisotropy is provided to birefringence by monoaxial orientation, thereby making it possible to reflect only a specific polarization component. Making use of this principle, for example, a reflection polarizing film which reflects P polarization and transmits S polarization can be designed. The desired birefringence is represented by the following expression (2).n1x>n2x,n1y=n2y  (2)wherein n1x and n2x are the refractive indices in the stretching direction of the respective layers, and n1y and n2y are the refractive indices in the direction perpendicular to the stretching direction of the respective layers.
JP-A 9-506837 and WO01/47711 disclose a biaxially oriented film consisting of high-refractive index layers made of polyethylene-2,6-naphthalene dicarboxylate (may be abbreviated as “PEN” hereinafter) and low-refractive index layers made of a thermoplastic elastomer and a monoaxially oriented multi-layer laminated film consisting of high-refractive index layers made of PEN and low-refractive index layers made of PEN comprising 30 mol % of isophthalic acid. These films are reflection polarizing films which reflect only specific polarization by using a resin having a positive stress optical coefficient in one type of layers and a resin having an extremely small stress optical coefficient, that is, a resin that rarely exhibits birefringence by stretching in the other type of layers. These reflection polarizing films can be used as a brightness improving film for liquid crystal displays.
In general, when the reflection polarizing film composed of a monoaxially oriented multi-layer laminated film is used as a brightness improving film for liquid crystal displays, it is desired that the visible light reflectance of P polarization which is a polarization component parallel to the plane including the stretching direction and a direction perpendicular to the film plane be close to 100% and that of S polarization which is a polarization component perpendicular to the above plane be close to 0%. Although the conventional reflection polarizing film can achieve a reflectance for P polarization with respect to the above plane of 90% or more, it has a reflectance for S polarization of about 10 to 15%. Therefore, a reduction in brightness is caused by a reflection loss in the liquid crystal display.
The above reflection polarizing film composed of a monoaxially oriented multi-layer laminated film has a visible light reflectance of S polarization with respect to the plane based on the stretching direction and the perpendicular direction of about 10 to 15%. This is mainly due to interfacial reflection caused by a difference in refractive index between a film substrate and air. To reduce the interfacial reflection, anti-reflection processing is employed.
As for anti-reflection coating used for displays, since the reflectance of the whole visible range cannot be reduced only with one coating layer, two coating layers consisting of a high-refractive index layer and a low-refractive index layer or three coating layers consisting of intermediate-refractive index layer, high-refractive index layer and low-refractive index layer are formed on the substrate.
Techniques for forming the above anti-reflection film are divided into a wet coating technique in which a solution is coated on a film and dried and this is repeated to form multiple layers and a dry coating technique in which sputtering/deposition is repeated. The former has a production problem that it is difficult to form multiple layers having a desired thickness accurately and the latter also has production problems that the production costs are high due to the use of a vacuum and that productivity is low though it has high accuracy.
For the above reasons, anti-reflection processing has never been carried out on the surface of a reflection polarizing film.